The present invention relates generally to cathodic protection, and more particularly, to an anticorrosive circuit in which a rectifier is interconnected between a buried structure to be protected from corrosion and a galvanic anode or low-resistant grounding metal or alloy buried in a suitably spaced apart relationship with the buried structure.
In general, buried structures such as underground gas or water pipes are subjected to corrosion by electrolytic action from unidirectional electric currents in the ground. These currents may result from galvanic couples in the ground, track returns in street-railway systems and electrified railroads, or a variety of other causes. Furthermore, the buried structures are also subjected to corrosion by electrolytic action from the electric currents flowing from anodes to cathodes of microcells or macrocells formed because of the local differences in the buried structures and the environments surrounding them.
The method of widest use for protecting the buried structures from electrolytic corrosion is cathodic protection. For instance, a galvanic anode made of a metal such as magnesium having a higher potential than the buried structure or low-resistive grounding metal or alloy equivalent in conductivity to the galvanic anode is buried in the vicinity of the buried structure, and is electrically connected thereto so that the anticorrosion current may flow from the galvanic anode or low-resistive grounding metal or alloy through the corrosive environment to the surface of the buried structure. In another cathodic protection method, an external current source is interconnected between a buried structure and an auxiliary electrode so that sufficient DC current may enter the entire surface of the structure. In the so-called "discharge system", the current leaking from the tracks of the electrified railroads or street-car systems is used. The point or points on the buried structure which is higher in potential that the tracks are electrically connected to the tracks so that the currents flow into the surrounding soil from the buried structure. From all of the considerations of the installation and maintenance including their cost, and safety, these cathodic protection methods are advantageous in that they may be applied almost under any environmental conditions; the maintenance after the installation is almost free, thus resulting in the considerable reduction in maintenance cost; the absolute magnitude of the flowing current is relatively low so that no interference problem will occur; and that the control of the potential of the buried structure relative to that of the surrounding soil may be relatively easily and correctly controlled. However, because of the reasons to be described hereinafter, the bove cathodic protection methods cannot be used under some environmental conditions. Especially in case of the first of the above three methods, even when the galvanic anode made of metallic magnesium is used, the potential difference between the buried structure and the galvanic anode is of the order of 0.6 to 0.7 volts (1.0 volt at the most) so that the potential difference most frequently tends to be cancelled by the stray currents. Thus, no effective anticorrosion current flows, and, in some cases, reverse current flows resulting in the corrosion of the buried structure. Furthermore, (1) each galvanic anode must provide the steady anticorrosion current of the order of 10 to 40 mA (more preferably 20 to 30 mA regardless of the different corrosive environmental conditions), and (2) in some cases, but not always, a reverse voltage of the order of tens volts at the maximum is impressed due to the stray currents in the ground so that the positive and effective cathodic protection cannot be attained unless the anticorrosion current flows even when the maximum reverse voltage is impressed. In order to solve these problems. it has been proposed to use semiconductor diodes available at the market. However, as is clear from their voltage-amper characteristic curves, when the reverse voltage is impressed across them, they may effectively prevent the reverse current; that is, corrosion current, but they cannot provide the sufficient and steady flow of forward current; that is, anticorrosion current. The reason is that with a conventional p-n diode, the effective anticorrosion current (forward current) flows only when the forward voltage of the order of 0.35 to 0.7 volts is impressed, but when the forward voltage is of the order of 0.1 to 0.2 volt, almost no forward current flows. Thus the conventional p-n diodes exhibit a poor forward current raising characteristic. As a result, the conventional p-n diodes cannot be used when the effective electromotive force or the effective difference in potential between the galvanic anode and the buried structure to be protected is unexpectedly low under some special environmental conditions which are very difficult to be investigated in advance.